501 MCQ Questions on DC Machine

151. For a D.C. shunt motor if the excitation is changed

(a) torque will remain constant.
(b) torque will change but power will remain constant.
(c) torque and power both will change.
(d) torque, power and speed, all will change.

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152. Which motor has the poorest speed control? [DSSSB JE – 2015]

(a) Differentially compounded motor
(b) Cumulatively compounded motor
(c) Shunt motor
(d) Series motor

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153. The plugging gives the

(a) zero torque braking.
(b) smallest torque braking.
(c) highest torque braking.
(d) none of the above.

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154. If the speed of a D.C. shunt motor is increased, the back e.m.f. of the motor will

(a) increase
(b) decrease
(c) remain same
(d) become zero

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155. Which motor should not be started on no-load?

(a) Series motor
(b) Shunt motor
(c) Cumulatively compounded motor
(d) Differentially compounded motor

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156. Ward-Leonard control is basically a

(a) voltage control method.
(b) field divertor method.
(c) field control method.
(d) armature resistance control method.

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157. For constant torque drive which speed control method is preferred? [DSSSB JE 2015]

(a) Field control
(b) Armature voltage control
(c) Shunt armature control
(d) Mechanical loading system

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158. In a D.C. generator all of the following could be the effects of iron losses except

(a) loss of efficiency.
(b) excessive heating of core.
(c) increase in terminal voltage.
(d) rise in temperature of ventilating air.

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159. The losses occurring in a D.C. generator are given below. Which loss is likely to have highest proportion at rated load of the generator? [DSSSB JE 2015]

(a) Hysteresis loss
(b) Field copper loss
(c) Armature copper loss
(d) Eddy current loss

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160. Which of the following loss in a D.C. generator varies significantly with the load current?

(a) Field copper loss
(b) Windage loss
(c) Armature copper loss
(d) None of the above

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161. A rotary converter in general construction and design, is more or less like

(a) a transformer.
(b) an induction motor.
(c) an alternator.
(d) any D.C. machine. [DSSSB JE – 2015]

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162. A rotary converter operates at a

(a) low power factor.
(b) high power factor.
(c) zero power factor.
(d) none of the above.

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163. In a rotary converter I2R losses as compared to a D.C. generator of the same size will be

(a) same.
(b) less.
(c) double.
(d) three times.

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164. For a ‘P’ pole machine, the relation between electrical and mechanical degree is

(a) θelec = 2θmech/P
(b) θelec = 4θmech/P
(c) θelec = θmech
(d) θelec = Pθmech/2

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165. Why is the armature core of a DC machine laminated?

(a) to reduce hysteresis loss.
(b) to reduce eddy current loss.
(c) to improve voltage regulation.
(d) to reduce armature reaction.

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166. Lap winding is suitable for

(a) low voltage low current dc generators.
(b) high voltage low current dc generators.
(c) high voltage high current dc generators.
(d) low voltage high current dc generators. [UPRVUNL JE 2016]

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167. In DC generator eddy current loss is nothing but

(a) iron loss.
(b) friction loss.
(c) windage loss.
(d) copper loss. [LMRC JE – 2016]

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168. An electrical generator is a machine which convert

(a) electrical energy to electrical energy.
(b) mechanical energy to mechanical energy.
(c) electrical energy to mechanical energy.
(d) mechanical energy to electrical energy. [UPRVUNL JE 2016]

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169. In a generator a dynamically induced emf is produced according to

(a) Faraday’s law.
(b) Biot Savart’s law.
(c) Ohm’s law.
(d) Kirchoff’s law.

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170. Compensating windings are used when loads are

(a) have small fluctuations.
(b) fluctuations of all kinds.
(c) steady.
(d) have large fluctuations.

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171. Use of fractional pitch winding

(a) make it stiffer.
(b) results in reduced leakage reactance.
(c) results in reduced axial length of the machine.
(d) all of the above.

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172. Where the shunt generators are widely used?

(a) Where the load is very high.
(b) Where a constant voltage is required over a narrow load range.
(c) Where often fluctuation is occurred.
(d) None of the above. [UP JAL NIGAM AE – 2016]

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173. What happens if the magnetic neutral axis coincide with the geometrical neutral axis in case of DC generator ? [JUVNL JE – 2017]

(a) The generator runs on designed speed.
(b) The generator runs on over load.
(c) There is no load on the generator.
(d) The generator runs at full load.

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174. The EMF induced in the armature of a shunt generator is 500 V. The armature resistance is 0.2 Ω if the armature current is 200 A. What is the terminal voltage? [JUVNL JE – 2017]

(a) 500 V
(b) 520 V
(c) 460 V
(d) 540 V

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175. Which of the following statement is WRONG regarding cumulative compound Motor? [UPPCL AE – 2016]

(a) Variable speed
(b) Low starting torque
(c) Speed control possible
(d) Adjustable varying speed

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176. Shunt generators are most suited for stable parallel operation as their voltage characteristics are

(a) drooping.
(c) linear.
(b) identical.
(d) rising. [Uttarakhand JE 2013]

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177. As we go from generator end towards load, the severity of a particular fault

(a) decreases.
(b) does not depend on location.
(c) remains same.
(d) increases. [Coal India Ltd. – 2017]

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178. The length of the air gap under the poles of a DC machine is not kept uniform so as to

(a) obtain a suitable main field flux.
(b) obtain better cooling.
(c) obtain sinusoidal armature mmf wave.
(d) minimize the effect of armature mmf on main field. [Coal India Ltd. – 2017]

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179. In a DC generator current to external circuit from armature is given through

(a) slip rings and brushes.
(b) commutator and brushes.
(c) solid connections.
(d) starter circuit.

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180. The machine which works on dynamically induced emf is

(a) dc motor.
(b) dc generator.
(c) transformer.
(d) induction motor. [LMRC JE – 2016]

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181. Iron losses in DC machine take place in

(a) yoke and armature only.
(b) armature core only.
(c) armature winding only.
(d) yoke only. [LMRC JE – 2016]

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182. Maximum efficiency of D.C. Machines occurs when

(a) machine is running at no load.
(b) core gets fully saturated.
(c) constant losses = losses proportional to square of current.
(d) constant losses = variable losses.

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183. In D.C. generator inter poles field coils are connected

(a) in series with armature winding.
(b) in parallel with armature winding.
(c) in series with load.
(d) in parallel with load. [UJVNL-2016]

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184. Match the following correctly

List I

  1. DC Series motor
  2. Squirrel-cage induction motor
  3. DC shunt motor

List – II

  1. Shearing and Pressing
  2. Haulage and Hoisting
  3. Rolling mill

(a) A-1, B-2, C-3
(b) A-2, B-3, C-1
(d) A-3, B-2, C-1
(c) A-3, B-1, C-2

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185. The polarity of a D.C. generator can be reversed by

(a) increasing field current.
(b) reversing the field current.
(c) reversing the field current & also direction of rotation.
(d) none of these. [UJVNL-2016]

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186. In case of four pole, lap wound machine if the air gap under each pole is the same, then what will be the result? [J & K JE – 2016]

(a) There will be reduced eddy current.
(b) There will be reduced hysteresis loss.
(c) Current in each path will not be the same.
(d) It will result in higher terminal voltage.

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187. Which loss has least proportion in DC machines? [J & K JE – 2016]

(a) Armature copper loss
(b) Field copper loss
(c) Magnetic loss
(d) Mechanical loss.

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188. A DC shunt generator supplies 450 A at 230 V. The resistances of shunt field and armature are 50Ω and 0.025Ω respectively. What will be the armature voltage drop? [J & K JE – 2016]

(a) 11.24 V
(b) 22.7 V
(c) 31.6 V
(d) 38.4 V

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189. In which of the following de motor the value of power drawn remains same at different loads? [UPRVUNL AE 2014]

(a) Cumulative compound
(b) Differential compound
(c) Series
(d) Shunt

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190. In case of 4-pole D.C. generator provided with a two layer lap winding with sixteen coils, the pole pitch will be

(a) 4
(b) 8
(c) 16
(d) 32 [ESIC JE – 2016]

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191. In a D.C. generator, the armature reaction results in

(a) demagnetisation of the centres of poles.
(b) magnetisation of interpole.
(c) demagnetisation of leading pole tip and magnetisation of the trailing pole tip.
(d) magnetisation of leading pole tip and demagnetisation of the trailing pole tip. [ESIC JE – 2016]

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192. The demagnitisation component of armature reaction in D.C. generator

(a) reduces generator e.m.f.
(b) increases armature speed.
(c) reduces interpole flux density.
(d) results in sparking trouble. [ESIC JE 2016]

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193. In DC generators, the polarity of the interpole is

(a) same as that of the main pole ahead.
(b) same as that of immediately preceding pole.
(c) opposite to that of the main pole ahead.
(d) neutral as these poles do not play part in generating emf. [ESIC JE 2016]

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194. In DC generators on no-load, the air gap flux distribution in space is

(a) sinusoidal.
(b) triangular.
(c) pulsating.
(d) flat topped. [ESIC JE – 2016]

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195. Flashing the field of DC generator means

(a) neutralising residual magnetism.
(b) creating residual magnetism by a dc source.
(c) making the magnetic losses of force parallel.
(d) increasing flux density by adding extra turns of windings on poles.

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196. In a DC machine stray loss is the sum of

(a) total copper loss and mechanical loss.
(b) armature copper loss and iron loss.
(c) shunt field copper loss and mechanical loss.
(d) iron loss and mechanical loss. [ESIC JE – 2016]

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197. Which one of the following is not the function of pole shoes in a DC machine?

(a) To reduce eddy current loss.
(b) To support the field coils.
(c) To spread out flux for better uniformity.
(d) To reduce the reluctance of the magnetic path.

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198. Match List I with List II and select the correct answer by using the given codes:

List-I Type of Generators

  1. Series wound
  2. Separately Excited
  3. Shunt wound
  4. Differentially Compounded

List-II Load Characteristics

  1. Rising
  2. Almost constant voltage
  3. Slightly dropping
  4. Rapidly falling

(a) A-4, B-3, C-2, D-1
(b) A-1, B-2, C-4, D-4
(c) A-3, B-2, C-4, D-1
(d) A-3, B-4, C-2, D-1

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199. If ‘t’ be the thickness of the lamination, then eddy current loss in a generator will vary to

(a) 1/t
(b) 1/t2
(c) t
 (d) t2 [ESCI JE 2016]

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200. In d.c. generator armature reaction is produced by

(a) field or armature current.
(b) field and armature current.
(c) field current.
(d) armature current. [FCI – 2015]

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